1. Field of the Invention
The invention refers to the field of ceramics and hard metal industry and relates to a process for the manufacture of hard metal component parts, in particular with complicated geometries, as they are for example used in machining tools and wear units.
2. Discussion of Background Information
Ceramic and powder metal parts with complicated geometries can be manufactured by performing mechanical reworking of the outer contours, beginning from raw bodies. With such materials, it is possible to execute the reworking process in a shaped state, a sintered state, or a state in between these two procedural steps. The cost of processing increases with the hardness of the material in its respective state, with the complexity of the geometry, and the number of pieces.
Large numbers of parts, composed of powder-technological or ceramic materials, are manufactured economically with the aid of end-for-like ("near net shape") thermoplastic casting processes. The injection molding of powder metal and ceramic is known, which is based on the mixing of a non-plastic powder with a thermoplastic synthetic binding agent and the occurrence of an injection molding mass, able to flow at higher temperatures and pressure, which can be processed in conventional synthetic material injection molding machines (U.S. Pat. No. 2,122,960, DE 680250).
Also known is a thermoplastic molding process that is based on the liquidization of non-plastic ceramic powders with the aid of paraffins and waxes. The molding process of thermoplastic suspensions is already possible at low pressures (SU 137807). The process is called hot casting or low pressure injection molding. Different ceramic materials, such as aluminum oxide, zirconium oxide, silicon carbide, have already been processed with this process. The material-specific characteristics are primarily based on the displacement of the thermoplastic binding agents that each consist of surface-active materials for the modification of the powder surface (SU 298566, SU 298567, DD 139397, DD 233117, DD 233119, SU 1590468). Concepts for a machine to realize the molding process are known (U.S. Pat. No. 4,416,603, DD 281913). An overview of the hot casting process is also published (Lenk, R., Technische Keramische Werkstoffe, Chapter 3.4.8.1).
Furthermore, from DD 286 311, a process is known for the manufacture of molded parts made of sintered materials using an injection molding process, whereby a plasticized mass is manufactured from a pre-treated metal powder and an organic binding agent and is then processed into molded parts after the removal of the binding agent using a subsequent injection molding process, and afterwards undergoes a sintering process. The binding agent consists of paraffin and polyethylene wax.
The powder-metallurgical injection mold process of heavy duty materials is furthermore known from Dropmann, et. al., Metall, vol. 45, no. 5, May 1991. The used binding agents thereby in essence determine whether or not an injection molding of the mass or a complete mold filling is possible. The viscosity of the masses thereby have a particular significance. Injection mold masses with a viscosity &lt;4000 mPa s are thereby not able to be used.
Up to now it was not possible to process hard metal materials with the hot casting process, since no stable, dispersed thermoplastic suspension could be produced due to the low viscosity of the thermoplastic binding agent (3 to 6 mPa s) and the high density difference between the hard metal powder and the used binding agent (12 to 14 fold). However, the manufacture of such stable, dispersed thermoplastic suspensions is the prerequisite for the manufacture of foundations with reproducible homogeneity and density, and thereby for the transformation to serial production.
Hard metal component parts with complex geometries, even with larger numbers, essentially have, until now, been manufactured with an expensive mechanical reworking in the sintered state.
In the last couple of years an injection molding technology has been developed that combines the possibilities of large-scale fabrication with the advantages of reproducing very complex geometries (DE 3808123.7). In order to minimize the problems of this process, resulting from the high density differences between the powder and the binding agent as well as from the very high flow velocities during the form filling process (segregation of mixtures and separation), synthetic material binding agents with higher viscosity are used. These temporary synthetic binding materials are expelled by extraction in a closed release process, with the aid of organic solvents before the component parts are sintered (Graf, W, et. al., Pulvermetall in Wissenschaft und Praxis, vol. 7, VDI-Verlag, Dusseldorf, 1991, pp.275-285).
The limits of this process lie in a complicated process sequence, high equipment costs that only amortize with a very large number of pieces, and the difficulty of using large quantities of organic solvents because of environmental issues.